This invention relates to cubic boron nitride (cBN) abrasive compacts.
Boron nitride exists typically in three crystalline forms, namely cubic boron nitride (cBN), hexagonal boron nitride (hBN) and wurtzitic cubic boron nitride (wBN). Cubic boron nitride is a hard zinc blend form of boron nitride that has a similar structure to that of diamond. In the cBN structure, the bonds that form between the atoms are strong, mainly covalent tetrahedral bonds. cBN is the second hardest material known to man and hence is a useful industrial material.
cBN has wide commercial application in machining tools and the like. It may be used as an abrasive particle in grinding wheels, cutting tools and the like or bonded to a tool body to form a tool insert using conventional electroplating techniques.
cBN may also be used in bonded form as a cBN compact, also known as PCBN (polycrystalline cBN). cBN compacts comprise sintered masses of cBN particles. When the cBN content is at least 70 volume % of the compact, there is a considerable amount of cBN-to-cBN contact. When the cBN content is lower, e.g. in the region of 40 to 60 volume % of the compact, then the extent of direct cBN-to-cBN contact is limited.
cBN compacts will generally also contain a binder which is essentially ceramic in nature. When the cBN content of the compact is less than 70 volume %, the matrix phase, i.e. the non-cBN phase, will typically also comprise an additional or secondary hard phase, which is usually also ceramic in nature. Examples of suitable ceramic hard phases are carbides, nitrides, borides and carbonitrides of a Group 4, 5 or 6 (according to the new IUPAC format) transition metal, aluminium oxide and mixtures thereof. Other additives typically metallic or intermetallic in nature, such as Ti, Al, Ni, W, Co or a combination thereof, may be added to improve bonding between these phases. The matrix phase is defined to constitute all the ingredients in the composition excluding CBN.
cBN compacts tend to have good abrasive wear due to the inherent high hardness of cBN crystals. In addition, they are thermally stable, have a high thermal conductivity, good impact resistance and have a low coefficient of friction when in sliding contact with a workpiece. The cBN compact, with or without a substrate (the substrate having been integrally bonded to the PCBN layer during the sintering process) is often cut into the desired size and/or shape of the particular cutting or drilling tool to be used and then mounted on to a tool body utilising brazing techniques.
cBN compacts may be mechanically fixed directly to a tool body in the formation of a tool insert or tool. However, for many applications it is preferable that the compact is bonded to a substrate/support material, forming a supported compact structure, and than the supported compact structure is mechanically fixed to a tool body. The substrate/support material is typically a cemented metal carbide that is bonded together with a binder such as cobalt, nickel, iron or a mixture or alloy thereof. The metal carbide particles may comprise tungsten, titanium or tantalum carbide particles or a mixture thereof.
A known method for manufacturing the polycrystalline cBN compacts and supported compact structures involves subjecting an unsintered mass of cBN particles together with a powdered matrix phase, to high temperature and high pressure (HPHT) conditions, i.e. conditions at which the cBN is crystallographically or thermodynamically stable, for a suitable time period.
Typical conditions of high temperature and pressure which are used are temperatures in the region of 1100° C. or higher and pressures of the order of 2 GPa or higher. The time period for maintaining these conditions is typically about 3 to 120 minutes.
cBN compacts with cBN content not exceeding 70 volume % are known as low CBN PCBN materials. Typically the cBN content of such compacts lies between 30 volume % and 70 volume %. Low cBN PCBN tools with reduced thermal conductivity are best suited for finishing operations (where the depth of cut is less than 0.5 mm) and for the machining of nodular cast irons. PCBN tool performance is generally dependent on the tool geometry, as well as to machining parameters such as cutting speed, feed and depth of cut; as well as the nature of contact. Continuous cutting would imply constant contact between the tool and the workpiece for prolonged periods of time; whereas intermittent contact is generally referred to as “interrupted cutting”.
The majority of machining applications contain combinations of the two types of cutting operations. In continuous cutting, temperatures at the cutting zone are much higher than in interrupted cutting, as the heat in latter can escape when the tool is not in contact with the workpiece, thus reducing overall temperatures. High temperatures result in higher “chemical wear” on the cutting tool typically identified as smooth and deep crater formation on the rake face of the tool. Therefore the main problem in continuous cutting is the deep crater formation as a result of chemical wear thereby reducing the overall strength of the cutting edge. However, in interrupted cutting operations the main problem is that the tools tend to fail catastrophically by fracturing or chipping due to cyclic impact conditions created by interrupted cutting. Cyclic mechanical, thermal stresses as well as general wear, (abrasive, adhesive and chemical) during the continuous cutting part of the interrupted cutting process lead to poor tool lives. Therefore in cutting operations that is a combination of both continuous and interrupted cutting the cutting tool edge tends to fail catastrophically by fracturing and chipping.
A conventional PCBN material design approach for manufacturing low cBN content PCBN materials has been to use metal-based starting materials such as Al, Ti and or intermetallic compounds of Ni, Ti with Al within the binder phase.
According to U.S. Pat. No. 4,334,928 at least one of the metals selected from Ni, Co, Fe and Cu can be added as a third component in the matrix to react with the cBN and secondary hard phase materials to form stable higher strength bonds. According to U.S. Pat. No. 4,693,746, the matrix phase may contain Ti-based compounds selected from the group of TiNz, Ti(C,N)z, TiCz, (Ti,M)Cz, (Ti,M) (C,N)z and (Ti,M)Nz (where M indicates a transition metal element of the group IVa, Va or VIa of the periodic table excepting Ti and z is within a range of about 0.7</=z</=about 0.85). The binder further contains about 20 to 30 weight % Al and about 5 to 20 weight % W. Again the aim of these further metallic species is to increase the bonding strength between matrix phase and CBN by reaction between Al, Ti and W-containing materials and cBN and forming high strength bonds.
The main drawback of these approaches is that the selection of an appropriate bonding aid material for both cBN and the secondary hard phase material is not easily achieved. This is primarily due to disparate nature of the materials involved.
For example, it is well known in the art that Al and Ti react with cBN to form reaction-bonding ceramic phases, however these metals do not easily produce high strength bonds with the secondary hard phase materials: transition metal carbides, nitrides and carbonitrides. Likewise, the addition of Ni, Co, Fe-type materials may help bonding between the secondary hard phase particles during sintering, but they do not provide for high strength bonding between cBN particles. A combined approach of using mixtures or alloys of these elements can be proposed, but this is also a sub-optimal solution because of the homogeneity constraints required in order to ensure that sufficient of each type of bonding agent is exposed to the particle type that it bonds best.
Therefore, prior art low cBN PCBN materials are typically sub-optimally bonded and hence do not perform well in demanding applications such as machining involving heavy interrupted cutting as well as the continuous cutting of hardened steels above HRc40.